Ataxia Telangiectasia (A-T) (Boder-Sedgwick/Louis-Bar syndrome) is an inherited immunodeficiency disorder with a prevalence of 1 in 30,000-100,000 births. A-T patients are characterized by pronounced facial spider veins (telangiectsia), recurrent sinopulmonary infections, and an irregular gate (ataxia), which results from progressive neuronal dysfunctions. These clinical presentations, secondary to the sensitivity to ionizing radiation and a marked predisposition to cancer, were explicated in 1995 by Savitsky et al. as an autosomal recessive mutation in the Ataxia Telangiectasia (ATM) gene. ATM is a 370 kDa nuclear Ser/Thr kinase contributing to the regulation of p53, BRCA1, and Chk2 signaling pathways, amongst others, and thus required for progression through mitotic checkpoints, double strand DNA repair, telomere repair, apoptosis, and meiosis. The central role of ATM in cell regulation suggests that loss of function mutations in ATM gene should result in broad pleiotropic effects. In preliminary studies, we have shown that targeted proteasome-mediated degradation is impaired in A-T cells, suggesting that reduce protein turnover in A-T cells could potentially contribute to the A-T phenotype (e.g. neurodegeneration). Our preliminary studies also demonstrated that the reduced protein turnover in A-T cells is associated with elevated expression of ISG15, the Interferon- Stimulated Gene 15, an ubiquitin-like protein proven to antagonize ubiquitin pathway. The present application is comprised of three Specific Aims designed to probe the role of ISG15 in Ataxia Telangiectasia. In the first specific aim, we will investigate the role of ATM in regulating ISG15 expression using various AT cell lines. In the second specific aim, we will test if the elevated expression of ISG15 impairs protein turnover in A-T neurons. Finally, in the third specific aim, we will examine if ISG15-mediated defect in ubiquitin-dependent protein degradation triggers neuronal cell death in vitro (using neurons grown in culture) and in vivo (in Atm -/- mice). The reason(s) for the progressive neurodegeneration in AT patients is not known. Results generated from the proposed experiments will provide insights in to the role of ATM/ISG15 in neurodegeneration in AT. The small molecule inhibitors targeting ISG15 pathway could then be developed in future to prevent ataxia associated with neurodegeneration in AT patients. Thus these studies have potential clinical applications for treating A-T patients. PUBLIC HEALTH RELEVANCE: We plan to test if the elevated expression of ubiquitin-like protein ISG15 is responsible for the neurodegeneration in Ataxia Telangiectasia (A-T). The proof-of-principle experiments described in this proposal will form the foundation for future development of small molecule inhibitors for the ISG15 pathway. These small molecule inhibitors are expected to prevent the ataxia in A-T patients.